• Geomechanics and Engineering
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• 제9권6호
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• pp.793-813
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• 2015

Characterization of discontinuous media is an endeavor that poses great challenge to engineers in practice. Since the inherent defects in cracked domains can substantially influence material resistance and govern its behavior, a lot of work is dedicated to efficiently model such effects. In order to overcome difficulties of material instability problems, one needs to comprehensively represent the geometry of cracks along with their impact on the mechanical properties of the intact material. In the present study, stress-strain results from laboratory experiments on Inada granite was used to derive crack tensor as a tool for the evaluation of fractured domain stability. It was found that the formulations proposed earlier could satisfactorily be employed to attain crack tensor via the invariants of which judgment on cracks population and induced anisotropy is possible. The earlier criteria based on crack tensor analyses were reviewed and compared to the results of the current study. It is concluded that the geometrical parameters calculated using mechanical properties could confidently be used to judge the anisotropy as well as strength of the cracked domain.

• 한국생산제조학회지
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• 제6권2호
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• pp.58-66
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• 1997

Ti alloys, with the advantageous tensile strength/density ratio and the chemical stability, have been used widely in the aerospace and chemical engineering industries and their usages are still expanding in various industrial areas. In the automotive industry, because of their superior merits of weight reduction and fuel saving, Ti alloys are expected to be used as various part materials including connecting rods, engine valves, springs and retainers, which are all subjected to the fatigue loads. In this study, using Ti-3A1-2.5V, the effects of temperature and microstructure change on fatigue crack propagation has been investigated. Five different microstructures were tested at the temperatures of room temperature, 20$0^{\circ}C$, 30$0^{\circ}C$ and 40$0^{\circ}C$ under the same frequency 20Hz. Some of the conclusions obtained are as follows: (1)Microstructurally, the morphology of less $\alpha$-phase and finer lamellar structure of $\alpha$ and $\beta$-Ti showed better registance to the fatigue crack propagation. (2)Fatigue crack growth rate increased with test temperature.

• 한국세라믹학회지
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• 제43권12호
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• pp.859-864
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• 2006

For the pressure compaction process of the ceramic powder, the green density is very different with both the ceramic body shape and the processing conditions. The density difference cause non-uniform shrinkages and deformations, and make cracks in the sintered ceramics. In this paper, Material properties of the alumina powder mixed with binder and the friction coefficient between the powder and the tool set were determined through the simple compaction experiments: Also the powder flow characteristics were simulated and the green density was analyzed during the powder compaction process with Finite Element Method (FEM). The results show that the density distributions of the green body were improved at the optimized processing condition and both the possibility of the farming crack generation and rho deformation of the sintered Alumina body were reduced.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 1999년도 춘계학술대회 논문집
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• pp.291-294
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• 1999

The effect of AI$_2$O$_3$+Y$_2$O$_3$additives on fracture toughness of $\beta$-SiC-ZrB$_2$composites by hot-pressed sintering were Investigated. The $\beta$-SiC-ZrB$_2$ ceramic composites were hot-presse sintered and annealed by adding 1, 2, 3wt% AI$_2$O$_3$+Y$_2$O$_3$(6:4wt%) powder as a liquid forming additives at 195$0^{\circ}C$ for 4h. In this microstructures, no reactions were observed between $\beta$-SiC and ZrB$_2$, and the relative density Is over 90.79% of the theoretical density and the porosity decreased with increasing AI$_2$O$_3$+Y$_2$O$_3$ contents. Owing to crack deflection and crack bridging of fracture toughness mechanism, the fracture toughness showed the highest of 5.5328MPa . m$^{1}$2/ for composites added with 2wt% AI$_2$O$_3$+Y$_2$O$_3$ additives at room temperature. But the standard deviation of fracture toughness of specimens decreased with increasing AI$_2$O$_3$+Y$_2$O$_3$ contents and showed the highest of 0.8624 for composite tilth 1wt%, AI$_2$O$_3$+Y$_2$O$_3$additives.

• Journal of the Korean Wood Science and Technology
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• 제37권4호
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• pp.293-299
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• 2009

섬유판을 이용한 탄화보드의 제조에 있어 무할렬 탄화법 및 탄화온도에 따른 수축률, 중량감소율 및 밀도변화에 대해 검토하였다. 섬유판을 두께별(3, 4.5, 6, 18 mm)로 준비한 후, 실험실용 탄화로를 사용하여 $400^{\circ}C{\sim}1,000^{\circ}C$의 온도조건에서 탄화보드를 제조하였다. 무할렬 탄화법은 시험편에 상하 눌림판을 설치하여 압체 탄화하는 방법을 적용하였다. 압체 탄화법에 의해 섬유판의 갈라짐과 뒤틀림 현상이 발생하지 않는 무할렬 탄화보드의 제조가 가능하였다. 탄화보드의 수축률은 길이방향 10~25%, 폭방향 12~25%, 두께방향 28~48%로 두께방향이 가장 크고 탄화온도가 상승함에 따라 커지는 경향이 나타났다. 탄화보드의 중량감소율은 섬유판의 두께에 따른 큰 차이는 나타나지 않았고 탄화온도가 상승함에 따라 커지는 경향이 나타났다. 탄화보드의 밀도는 두께 3 mm의 경질 섬유판에서 가장 컸고 탄화온도가 상승함에 따라 커지는 경향이 나타났다.

• 대한기계학회논문집
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• 제15권6호
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• pp.2002-2014
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• 1991

본 연구에서는 모재균열의 열림변위로 인한 변형을 고려하여, 모재균열 주위 의 응력분포를 구하기 위한 2차원 해석방법을 제안한다. 제안된 방법은 두께방향 다 항식 형태로 가정된 변위성분으로부터 모재균열 주쥐의 변위, 응력분포를 구한다. 본 방법은 적층판의 프아송비(Poisson's ratio) 효과와 열잔류응력(thermal residual stress)의 영향을 고려하였으며, 계면층(interface layer) 개념을 사용하여 특성손상 상태 이후에 발생하는 층간분리를 평가하기 위한 기초자료인 층간수직응력과 층간전단 응력을 결정하였다. 제안된 방법의 타당성을 검증하기 위하여, 유한요소해석(finite element analysis)을 수행하여 제안된 방법의 응력분포 결과를 유한요소해석 결과와 비교하여 보았다.

• 한국염색가공학회지
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• 제9권1호
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• pp.15-22
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• 1997

The alkali treated and black dyed PET fabrics were sputter etched under Ar gas atmosphere. The color depth of PET fabrics were increased with sputter etching time only under some limits of discharge power. And above that limits of discharge power the color depth of PET fabrics was decreased by increasing discharge power and treatment time. Minute cracks were made by sputter etching in the whole surface of fabrics to the direction of perpendicular to the longitudinal side of yarns. The fineness and density of minute crack were increased with lowering discharge power. And the size of crack was far smaller than that of microcrater which was obtained by alkali treatment. It is considered that the increase of color depth is related to the minute crack. The water permeation time of sputter etched fabrics was increased with increasing discharge power and treating time. The increase of color depth attained by sputter etching was fully kept through repeated laundering.

• Journal of Mechanical Science and Technology
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• 제18권8호
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• pp.1375-1387
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• 2004

A solution is given for the elastodynamic problem of a crack perpendicular to the graded interfacial zone in bonded materials under the action of anti plane shear impact. The interfacial zone is modeled as a nonhomogeneous interlayer with the power-law variations of its shear modulus and mass density between the two dissimilar, homogeneous half-planes. Laplace and Fourier integral transforms are employed to reduce the transient problem to the solution of a Cauchy-type singular integral equation in the Laplace transform domain. Via the numerical inversion of the Laplace transforms, the values of the dynamic stress intensity factors are obtained as a function of time. As a result, the influences of material and geometric parameters of the bonded media on the overshoot characteristics of the dynamic stress intensities are discussed. A comparison is also made with the corresponding elastostatic solutions, addressing the inertia effect on the dynamic load transfer to the crack tips for various combinations of the physical properties.

• 대한기계학회논문집A
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• 제29권10호
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• pp.1336-1343
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• 2005

The detection mechanism of the flaw for the nondestructive testing using eddy current is related to the interaction of the induced eddy currents in the test specimen with flaws and the coupling of these interaction effects with the moving test probe. In this study, the two-dimensional electromagnetic finite element analysis(FEM) fur the eddy current signals of the aluminum plate with different depth of surface cracks is described and the comparison is also made between experimental and predicted signals analyzed by FEM. In addition, the characteristics of attenuation of the eddy current density due to the variation of the depth of a conductor are evaluated. The effective parameters for the application of eddy current technique to evaluate surface cracks are discussed by analyzing the characteristics of the eddy current signals due to the variation of crack depths.

• Nuclear Engineering and Technology
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• 제51권2호
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• pp.479-489
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• 2019

Low-cycle fatigue (LCF) tests were performed for Alloy 690 and 316 SS in a simulated pressurized water reactor (PWR) environment. Alloy 690 showed about twice longer LCF life than 316 SS at the test condition of 0.4% amplitude at strain rate of 0.004%/s. Observation of the oxide layers formed on the fatigue crack surface showed that Cr and Ni rich oxide was formed for Alloy 690, while Fe and Cr rich oxide for 316 SS as an inner layer. Electrochemical analysis revealed that the oxide layers formed on the LCF crack surface of Alloy 690 had higher impedance and less defect density than those of 316 SS, which resulted in longer LCF life of Alloy 690 than 316 SS in a simulated PWR environment.